Low cost, low power consumption, excellent noise characteristics and modulation precision are generally required to PLL modulation. When modulation is carried out by PLL, it is more desirable that the width of the frequency band of PLL (PLL band) is set to be larger than the width of the frequency band of modulated signals (modulation band) to enhance the modulation precision.
However, if the PLL bandwidth is increased, it would cause degradation of noise characteristics. Therefore, there has been proposed a technique called as two-point modulation in which the PLL bandwidth is set to be narrower than the modulation bandwidth, and modulation within the PLL band and modulation out of the PLL band are applied at two different places (for example, see Patent Document 1).
FIG. 10 is a schematic diagram showing a conventional broadband modulation PLL. As shown in FIG. 10, the conventional broadband modulation PLL is equipped with PLL containing a voltage controlled oscillator (hereinafter referred to as VCO) 1 whose oscillation frequency is varied in accordance with a voltage input to a control voltage terminal, a frequency-divider 2 for dividing the frequency of an RF modulation signal output from VCO1, a phase comparator 3 for comparing the phase of an output signal of the frequency-divider 2 and the phase of a reference signal and outputting the signal corresponding to the phase difference and a loop filter 4 for averaging the output signal of the phase comparator, a modulation sensitivity table 7 for outputting a modulation signal on the basis of modulation data, a D/A converter 10 for converting the output signal of the modulation sensitivity table 7 to an analog voltage while adjusting a gain in accordance with a gain control signal from a controller 6, a delta sigma modulator 9 for applying delta sigma modulation to a signal achieved by adding the output signal of the modulation sensitivity table 7 and channel selecting information and outputting the addition result as a frequency-dividing ratio to the frequency-divider 2, and an A/D converter 11 for converting the input voltage value to a control terminal Vt to a digital value and outputting the digital value thus converted to an A/D converter 11.
It is generally needed in the two-point modulation that the timing is coincident between two points, and if any timing difference occurs between the two points, modulation precision such as EVM (Error Vector Magnitude) or the like is degraded. Under the state that there is no timing difference between the two points, EVM is equal to zero. If the difference (the time lag between two points) is larger, EVM is increased (degraded).
However, with respect to the conventional frequency synthesizer described above, there is no description concerning a specific method as to how to perform this timing adjustment, and thus it is difficult to set a proper timing.
(Patent Document 1) Specification of U.S. Pat. No. 6,211,747
The present invention has been implemented in view of the foregoing situation, and has an object to provide a broadband modulation PLL having enhanced modulation precision, a timing error correcting system for the broadband modulation PLL, a timing error correcting method and a method of adjusting a radio communication device having the broadband modulation PLL.